Zeroing in on Black Holes
By Linda Copman, based on an interview with Andrea Ghez
Animations and images created by Dr. Andrea Ghez and her research team at UCLA from data sets obtained with the W. M. Keck Telescopes
“The larger the telescope the more detail, or the closer to the center of the Galaxy I can see. Having access to the Keck Telescopes - the largest telescopes in the world - was KEY to my success in studying the black hole at the Galactic Center. So was having high angular resolution techniques - now in the form of adaptive optics - which allow this resolution to be achieved. Without AO the atmosphere blurs the images and you lose the advantage of having a large telescope.”
- Dr. Andrea Ghez
An object becomes a black hole when it is compressed to the point that gravity overcomes all other forces. While black holes don’t have a physical size, there is a radius associated with them, called the Schwartzschild radius or “R_s.” This radius is not a physical size or surface. Rather, it is the last point at which light can escape the gravitational pull of the black hole, and therefore the last point from which scientists can gather any information. Scientists call this point the
It is also the radius to which one would have to compress an object for it to become a black hole. At this point, gravity would prevail over all other known forces, and the object would collapse to an infinitesimally small scale and become a black hole. It turns out that the size of the object when this happens is the same as the event horizon. Thus, the event horizon of a black hole is also its Schwartzschild radius, or “R_s.”
To prove the existence of a black hole, which doesn’t emit light like a star does, scientists apply Kepler’s Laws to track the orbits of neighboring stars. By tracking the objects which orbit around a proposed black hole, it is possible to determine how much mass is confined within this black hole and its associated Schwartzschild radius.
Sgr A*, the black hole at the center of the Milky Way Galaxy, was recognized to be an unusual radio source about 30 years ago, but it wasn’t known to be a black hole back then because there was no way to say how massive the object was.
Dr. Andrea Ghez’s utilized the powerful telescopes at Keck Observatory to conduct a groundbreaking experiment and prove the existence of a supermassive black hole at the Galactic Center of the Milky Way. Ghez tracked the orbits of stars in the vicinity of Sgr A* to measure its mass and then confined that mass to within a specific radius. Ghez’s work was critical in establishing that the radio source was most likely caused by light arising from matter falling onto a black hole. Astronomers can’t see a black hole directly, but they can see matter falling toward the black hole. The radio source was not the black hole itself, but matter just outside the event horizon seen falling towards Sgr A*.
As a result of Ghez’s work at Keck Observatory, we now know much more about Sgr A*, as well as about black holes in other galaxies. “Sgr A* is one of the least massive among the supermassive black holes in the universe. “It’s also not accreting a lot of matter, so the light associated with the matter falling toward the black hole is fairly dim,” explains Ghez.
Massive stars, or stars whose mass is more than 20 to 30 times the mass of the Sun, will end their lives by turning into “little” black holes. These “little” black holes have a mass of roughly 3 to 10 times the mass of the Sun.
In contrast, the supermassive black holes at the center of galaxies are typically a million to a billion times the mass of the Sun. Such supermassive black holes were not predicted by theory, as were the “little” black holes, but were found observationally.
Scientists are still trying to understand how supermassive black holes form. The consensus today is that these huge black holes came into existence when their galaxies were first forming. “We noticed that the mass of a black hole is related to the central spherical component of its galaxy,” says Ghez. “These two things are of such different scales that the only way we know how to induce a correlation between them is for them to be the outcome of the same process. What this means is that we think they were born that way,” Ghez speculates.
Black holes, with their seemingly improbable properties, have become iconic in popular culture. Comparisons between energy-sucking activities, such as certain kinds of jobs, and black holes are commonplace. The concept of something which pulls everything in its vicinity into a deep, dark abyss has captured our imaginations and, as frequently happens in such instances, misconceptions about black holes abound. In order to debunk some of these misconceptions, Dr. Ghez provides the following list of the real attributes of black holes:
- Black holes don’t expand with time. However, they might gain mass by swallowing up stars and local gas from their local surroundings.
- Supermassive black holes will probably “live” forever. They are quite stable.
- Supermassive black holes are likely found in every galaxy. They can effect the formation and evolution of stars in their environment. They also provide an important clue as to how galaxies might form.
Ghez’s research continues to zero in on black holes, particularly on Sgr A*, the closest black hole to home. Ghez is currently studying three aspects of Sgr A* that she hopes will elucidate our understanding of these fascinating objects:
1. Subtle variations in the orbits of the stars around Sgr A* that the theory of general relativity (GR) predicts: Such observations could provide a test of GR, which is the least tested of the four fundamental forces.
2. Understanding star formation: Sgr A* is a hostile environment for star formation, and yet astronomers keep discovering more and more young stars, closer and closer to the black hole.
3. Understanding the accretion process, evolution, and nature of our Galaxy’s black hole: Ghez, like many other astronomers, would like to know how black holes are able to accrete without emitting too much energy, what their spin is, and the biggest mystery of all: what the physical description of a black hole is inside the event horizon.
Ghez admits that the first moon landings had a big impact on her. She attributes her success as an astronomer, at least in part, to a few great mentors that she encountered during her years in school. In particular, Ghez recalls her chemistry teacher Judith Keane, the only female science teacher she had in high school or college; Hale Bradt, an MIT professor whom she worked with as an undergraduate on “little” black holes; and Gerry Neugebauer, her Caltech Ph.D. advisor.
“Neugebauer was the best advisor I could have had - very supportive, very straight with me, and he gave me lots of independence,” Ghez says. “I love the research process. First you have to figure out what might be an interesting question to ask given what is currently known and possible to do. I really enjoy using new techniques. While this can be a struggle because things are not totally ironed out, you get to see things in a way that have never been seen before.”
Ghez has two children with whom she spends most of her non-working hours. To relieve stress and maintain her perspective, she swims with a masters swim club. When asked what she likes least about her work as an astronomer, she was hard pressed to think of anything negative. “I can’t think of anything bad about being an astronomer “I feel so fortunate to be in this line of business,” says Ghez. Ghez loves being able to pursue questions that she is curious about, and she loves being able to work on problems that she defines and feels passionate about.
Ghez admits that part of the reason she teaches at UCLA is because this enables her to earn time on the Keck Telescopes. Keck time is very precious, and Ghez has passed up family vacations, high school reunions, lots of holidays, and even a wedding anniversary in order to work at Keck Observatory.
“Keck is an absolute gem! It houses the largest optical/infrared telescopes in the world. This allows astronomers who use Keck all sorts of advantages. We can see more distant objects, and therefore further back in time, so we learn more about the early universe. We can also see in more detail. This is what allowed us to demonstrate the existence of a supermassive black hole at the center of the Galaxy.”
- Dr. Andrea Ghez
Planned improvements in instrumentation, and in particular a more advanced adaptive optics system, will greatly improve Keck Observatory’s views of the center of the Galaxy. The Next Generation Adaptive Optics (NGAO) system proposed for the Observatory will improve the sensitivity of the orbital measurements Ghez and other astronomers are able to make. Such measurements will enable scientists to test the theory of general relativity, study star formation in the vicinity of a black hole, and understand how a black hole gains weight.
Einstein’s theory of relativity predicts the existence of black holes, as a place where there is a singularity of the warping of the space-time continuum. Since density rises to infinity in a black hole, they warp the space-time continuum. Yet our current theory of gravity clearly breaks down within the limits of a black hole. “Presumably, we’ll understand black holes when we can make the study of things that are very small (quantum mechanics) work together with the study of things that have strong gravity (general relativity),” explains Ghez. That’s why Ghez and other astronomers want to study gravity in a strong gravity regime, such as a black hole, where there have been very few tests of the theory.
“We’ll get a better understanding of how our physical world works. We simply don’t have a perfect description or understanding today. And an improved understanding is likely to yield insight into other questions about the origin and evolution of our universe,” says Ghez.
Visit the Monsters of the Milky Way home page for clips from a 2006 NOVA documentary on the supermassive black hole at the center of our Galaxy. This site includes interviews with experts in the field and stunning digital simulations of black holes.